As the electronics industry advances toward higher circuit densities, efficient thermal management will assume increasing importance. The removal of heat from critical circuit components through the circuit substrate is directly dependent on the thermal conductivity of the substrate. Beryllium oxide (BeO) has traditionally been the ceramic of choice for applications requiring electrically insulating materials having high thermal conductivity. Unfortunately, beryllium oxide is toxic to a small fraction of the general population, thus leading to a significant reluctance to use it.
Alumina (Al.sub.2 O.sub.3) is nontoxic and is easily fired to full density at 1500.degree.-1600.degree. C.; however, its thermal conductivity of between about 20 to about 30 W/m.degree.K is about one order of magnitude less than that of BeO (which has a thermal conductivity of about 260 W/m.degree.K). Additionally, the coefficients of thermal expansion (CTE) over the range of 25.degree.-400.degree. C. for alumina (6.7.times.10.sup.-6 /.degree.C) and beryllia (8.0.times.10.sup.-6 /.degree.C) are not well matched to those of semiconductors such as silicon (3.6.times.10.sup.-6 /.degree.C), and gallium arsenide (5.9.times.10.sup.-6 /.degree.C). Thus, alumina and beryllia provide less than ideal results when used in applications such as integrated circuit substrates through which heat transfer is to occur. In contrast, the CTE for aluminum nitride (AlN) is 4.4.times.10.sup.-6 /.degree.C, a value which is well matched to both of the previously described semiconductor materials.
In addition to having a CTE which makes it compatible with materials such as silicon and gallium arsenide, AlN can be sintered to provide shaped ceramic articles. Additionally, AlN articles are amenable to a variety of metallization processes. As such, AlN has repeatedly been suggested as a ceramic substrate for semiconductor applications. Although a variety of attempts to produce sintered AlN parts having high thermal conductivity are described in the literature, these generally have achieved limited success.
There is extensive literature on the sintering of AlN using a variety of sintering or densification aids. The bulk of the literature centers around the use of oxides of either rare earth elements (i.e., yttrium and lanthanide series elements), oxides of alkaline earth elements (i.e., the Group IIA elements), and mixtures thereof. These include compounds such as Y.sub.2 O.sub.3, La.sub.2 O.sub.3, CaO, BaO, and SrO. A system using Y.sub.2 O.sub.3 and carbon is described in a variety of patents, such as U.S. Pat. No. 4,578,232, U.S. Pat. No. 4,578,233, U.S. Pat. No. No. 4,578,234, U.S. Pat. No. 4,578,364 and U.S. Pat. No. 4,578,365, each of Huseby et al.; U.S. Pat. Nos. 3,930,875 and 4,097,293 of Komeya; and U.S. Pat. No. 4,618,592 of Kuramoto. Additionally, there is a wide variety of patents using Y.sub.2 O.sub.3 and YN including U.S. Pat. No. 4,547,471 of Huseby et al.
In the Huseby et al. patents which relate to the Y.sub.2 O.sub.3 and carbon system, described above, AlN samples which are doped with Y.sub.2 O.sub.3 and carbon are heated to 1500.degree.-1600.degree. C. for approximately one hour. The carbon serves to chemically reduce Al.sub.2 O.sub.3 phases contained in the AlN, thereby producing additional AlN and lowering the overall oxygen level in each part. The patents state that the Y.sub.2 O.sub.3 sintering aids are unaffected by this process. The parts are then sintered at about 1900.degree. C. Thermal conductivities as high as 180 W/m.degree.K have been reported for carbon treated samples produced by the methods described in these patents. Some evidence indicates, however, that these methods may introduce residual, free carbon within the sintered AlN piece, and this residual carbon can act to decrease the dielectric constant and loss throughout the piece. These effects may be undesirable in electronic applications, although acceptable in many other applications. Additionally, two other patents of Huseby et al. (U.S. Pat. No. 4,478,785 and U.S. Pat. No. 4,533,645) disclose a similar process that does not make use of a Y.sub.2 O.sub.3 sintering aid.
Other techniques for the production of sintered AlN and high thermal conductivity AlN have also been disclosed. See, for example, U.S. Pat. No. 4,659,611 of Iwase et al., U.S. Pat. No. 4,642,298 of Kuramoto et al., U.S. Pat. No. 4,618.592 of Kuramoto et al., U.S. Pat. No. 4,435,513 of Komeya et al., U.S. Pat. No. 3,572,992 of Komeya et al., U.S. Pat. No. 3,436,179 of Matsuo et al., European Patent Application No. 75,857 of Tsuge et al., and U.K. Patent Application 2,179,677 of Tanicjuchi et al.
Thermal conductivities of up to 200 W/m.degree.K have been reported in parts sintered from mixtures of 1-5% Y.sub. O.sub.3 and an aluminum nitride powder containing a low oxygen level (for example, an oxygen content less than 1.0%). See, for example, K. Shinozaki et al., Seramikkusu, 21(12):1130 (1986). In a presentation at the 89th Annual Meeting of the American Ceramic Society, (Pittsburgh, Pa., May 1987), Tsuge described a three stage process for increasing the thermal conductivity of sintered AlN parts. Further treatment of these parts for as long as 96 hours to remove the yttrium aluminate grain boundary phase reportedly can increase the thermal conductivity to 240 W/m.degree.K. Finally, by treating these samples to increase the average grain size, thermal conductivities which approach the theoretical thermal conductivity of 320 W/m.degree.K have been reported. This method, however, requires lengthy, multiple, independent steps to increase the thermal conductivity of the aluminum nitride material and produces sintered parts having large grain sizes. Additionally, the ultra-high thermal conductivity samples which have been produced to date contain extremely low levels of both oxygen (less than 400 ppm) and yttrium (less than 200 ppm).
Finally, German Patent DE 3,627,317 to Taniguchi et al. describes the use of mixtures of alkaline earth and rare earth halides and oxides to produce aluminum nitride parts that are reported to have thermal conductivities as high as 250 W/m.degree.K.
This technology, however, has been demonstrated only with parts which are relatively thick (e.g., 6 mm or more). Thin samples, (on the order of 3 mm) such as those associated with circuit substrate applications exhibit significantly lower thermal conductivities, e.g. 170-205 W/m.degree.K.
None of the processes described above teach the production, via a simple firing program, of sintered AlN parts having a thickness below about 6 mm and a thermal conductivity above about 220 W/m.degree.K.
Additionally, each of the processes described above requires either the use of solid-phase carbon or carbonaceous compounds, or extended firing schedules to increase the thermal conductivity of the final sintered part. The use of solid-phase carbon or carbonaceous compounds interferes with the ability to increase the thermal conductivity of previously sintered aluminum nitride parts, and has the potential for leaving porosity in AlN greenware following the heat treatment step. This porosity may result in non-uniform sintering.
Since AlN is a material with a number of unique properties which render it particularly useful in electronic and structural applications, it is particularly desirable to develop a method for the production of high thermal conductivity aluminum nitride which is simple and does not require extremely long firing times to produce a dense article.